In known tools of this type, speed regulating devices are provided which are constructed as discrete circuits. These circuits include a potentiometer so that the speed of the tool can be regulated by the operator. The actual speed control is carried out by measuring the voltage across the motor and utilizing this voltage to vary the threshold of a switch controlling the "on" time of a thyristor in the motor circuit. The "on" time of the thyristor in each cycle varies as a function of the threshold of the switch so that the angle of current flow is changed until the actual speed of the motor corresponds to the selected speed. Since the threshold of the above-mentioned switch tends to vary because of manufacturing tolerances, the actual speed of the motor may, in fact, differ quite considerably from the selected speed. While this may be acceptable for many applications, this error in speed cannot be tolerated for higher quality, more expensive tools.